1. Field of the Invention
The present invention relates to an image reading apparatus. More particularly, the present invention relates to a line image scanner for optically reading a document sheet in full color. The present invention also relates to an image sensor chip advantageously used for such an image scanner.
2. Description of the Related Art
An example of conventional image reading apparatus is shown in FIG. 14 of the accompanying drawings. The illustrated reading apparatus includes a light source unit Ba made up of a cold-cathode tube 1' and a mirror 2'. The conventional apparatus also includes an inverter unit Bb for providing driving power to the cold-cathode tube 1' via a flexible cable 3', and a light leading unit Bc provided with mirrors 4a', 4b'. The conventional apparatus further includes an image reading unit Bd provided with a lens 5' and an image sensor 6'.
In operation, the light source unit Ba is reciprocated in the secondary scanning direction under a stationary glass plate 7' on which a document sheet K to be read out is placed. Accordingly, the light leading unit Bc is repeatedly moved in the secondary scanning direction.
The conventional image reading apparatus has been found to be disadvantageous in the following points.
First, the light source unit Ba, the inverter unit Bb and the light leading unit Bc are produced separately from each other. With such an arrangement, it is difficult to accurately position these units to each other. Further, production costs tend to be high since a plurality of separate units need to be manufactured.
Still further, the flexible cable 3' connecting the light source unit Ba to the inverter unit Bb is long enough, so that the reciprocating movement of the light source unit Ba is not hindered. However, as the length of the flexible cable 3' increases, the loss of the driving power provided by the inverter unit Bb to the cold-cathode tube 1' increases. As a result, the luminance of the cold-cathode tube 17 may be unduly reduced.
The conventional image reading apparatus also has the following disadvantage.
Though not shown in FIG. 14, the image sensor 6' includes a plurality of image sensor chips. Each image sensor chip is formed with three rows of light receiving elements extending in the primary scanning direction. A first row is made up of red light receiving elements used for selectively detecting a red component of white light. Similarly, a second row is made up of green light receiving elements used for selectively detecting a green component of white light, while a third row is made up of blue light receiving elements used for selectively detecting a blue component of white light.
Each row of light receiving elements has a pitch P between the light receiving elements in the primary scanning direction. The pitch between the light receiving elements in the secondary scanning direction (which is perpendicular to the primary scanning direction) is also P. As viewed in the secondary scanning direction, each of the light receiving elements has a length of P/2.
For providing color selectivity, color filters are used for the light receiving elements. Specifically, each red light receiving element is covered by a red filter which allows selective permeation of red light, whereas each green light receiving element is covered by a green filter which allows selective permeation of green light. Similarly, each blue light receiving element is covered by a blue filter which allows selective permeation of blue light.
In the conventional image reading apparatus, no attention has been paid to e.g., the thickness of the color filters, and three types of color filters may have the same thickness. With such an arrangement, however, it may be impossible to realize a high-fidelity reproduction of the read image. This is partly because properties of the color filters are not the same for the different color lights (red, green, blue) and partly because properties of the light receiving elements are not the same for the different color lights, either.
Specifically, as shown in FIG. 15, among the three colors (Red, Green and Blue), a conventionally available light receiving element (e.g., phototransistor) has the highest relative sensitivity for red, the second highest relative sensitivity for green, and the lowest relative sensitivity for blue.
FIG. 16 shows relationship between the wave length of incident light and the transmittance of the respective color filters (Red filter, Green filter and Blue filter). As illustrated, the red filter has the highest transmittance, while the green filter has the second highest transmittance, and the blue filter has the lowest transmittance.
As shown in FIG. 17, a typical cold-cathode tube generates white light whose green component has the highest energy ratio compared with those of the red and blue components.
FIG. 18 shows the reflectivity of red (R), green (G) and blue (B) lights on four types of test charts (WHITE, RED, GREEN, BLUE and BLACK test charts). As illustrated, of three colors, blue light is reflected most on the white test chart. Red light is reflected most on the red test chart, green light is reflected most on the green test chart, and blue light is reflected most on the blue test chart. When the test chart is black, the three color lights are hardly reflected.
FIG. 19 is obtained from a combination of FIG. 16 and FIG. 18. As is shown, when using the white test chart, the green light has the highest product of the transmittance and the reflectivity.
FIG. 20 is obtained from a combination of FIG. 15, FIG. 17 and FIG. 19. FIG. 20 shows the output voltages generated by the three types of light receiving elements (Red, Green and Blue) when the four types of test charts (WHITE, RED, GREEN, BLUE and BLACK) are irradiated with white light. As is shown, when using the white test chart, the green light receiving element generates the highest output voltage. When using the red test chart, the red light receiving element generates the highest output voltage.
Still further, the conventional image reading apparatus is disadvantageous in the following point.
Referring to FIG. 21, for performing image reading for one line, the light receiving elements 8' (only one shown) of each image sensor chip are advanced by the distance P in the secondary scanning direction with respect to the document sheet. During this movement, however, the light receiving element 8' scans a rectangular area having a length of (P+L). This means that image reading for each line is performed for an unduly larger area due to the length L of the light receiving element itself. As a result, with the use of the conventional image reading apparatus, a high-fidelity printout reproduction of the image carried by the document sheet may not be realized.